DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 379 



Under the influence of enzymes produced by molds, yeasts and 

 bacteria the fatty acids occurring as glycerides are decomposed into 

 glycerin and fatty acids. The extent of fat decomposition, brought 

 about largely by molds in the opinion of some, is shown by numerous 

 experiments with peanut cake, olive press cake, cottonseed meal, 

 almond oil, corn meal, etc. In a number of these experiments Asper- 

 gillus niger seemed to be particularly efficient in decomposing fats. 

 Analogous decomposition processes may occur in the soil as proved by 

 the experiments of Rubner. 



ORGANIC ACIDS 



Source. The cleavage products of proteins include large quantities 

 of amino-acids. The latter are still further transformed and yield a 

 variety of fatty acids. The carbohydrates being present in larger 

 quantities than the proteins are still more important as a source of 

 organic acids. Finally, the fats, gums, and higher alcohols contribute 

 additional quantities of the latter. Among the more simple acids, 

 acetic, propionic, butyric, succinic and lactic are common. The extent 

 of acid production was already indicated in connection with cellulose 

 decomposition by the methane and hydrogen bacilli. Apart from these 

 organisms, organic acids are formed by nearly every important species 

 of soil bacteria; moreover, the tissues of dead plants and animals are 

 not the sole source of organic acids in the soil. According to Stoklasa 

 conditions may occasionally occur in the latter, especially when 

 atmospheric oxygen is excluded, that favor the excretion by plant roots 

 of appreciable quantities of acetic acid. 



Aside from the organic acids produced by bacteria, we must also 

 consider the acids produced by molds; among these oxalic and citric 

 acids are most important. Certain members of the Aspergillus niger 

 group are able to convert as much as 40 per cent of the sugar in solution 

 into citric acid ;' the latter is then further oxidized into oxalic acid. In 

 addition to the Aspergilli, several Penicillia, Mucors, Absidia and other 

 molds, which have been isolated from the soil, are able to produce citric 

 or oxalic acids, or both. The acid produced in the culture medium is 

 either allowed to accumulate or is further oxidized. Aspergillus niger 

 oxidizes sugar first into citric acid, the latter is then oxidized to oxalic 

 acid and finally to carbon dioxide. 



